Referring to FIG. 1, there is a height difference between an active area and a shallow trench area before a polysilicon gate begins to grow. The height difference will make the surface of the polysilicon gate after growth uneven. The uneven polysilicon gate surface will cause a series of problems to the subsequent processes, particularly to integrated circuit products followed by a SiGe epitaxial growth, SiGe residue may be present above the protuberance of the polysilicon gate. Thus, the protuberances should be removed and the polysilicon gate surface should be flat.
The protuberances on the surface of the polysilicon gate can certainly be removed by chemical mechanical polishing, but a special slurry is needed to polish the polysilicon material and a thickness difference of the polysilicon gate will be increased remarkably, that is to say, although the some parts of the protuberance can be removed, the thickness uniformity of the polysilicon gate on the whole wafer will become poor.
Additionally, an oxide layer is formed on the surface of the silicon oxide layer; flat oxide film can be obtained by either the process of the flowably chemical vapor deposition (FCVD) or the process of the spin coating. Referring to FIG. 2, FIG. 2 is a view of the microscopic structure of the silicon oxide film deposited by FCVD, wherein the silicon oxide film is deposited faster on a trench area, thus the surface of the silicon oxide film obtained is flatter than the surface of the polysilicon gate layer.
Therefore, a technical solution for removing or eliminating the uneven surface of the polysilicon gate is expected to be provided, so as to further avoid a series of problems resulted from the uneven surface of the polysilicon gate in the subsequent processes.